I was really excited about JPEG 2000 when I first heard about it in the mid 2000s. As is tradition it was pretty much killed by the possibility of patents that weren't part of the industry agreement.
JPEG2000 just wasn't a great format either. Wavelets don't compress well because they get blurry, which isn't pleasant to look at, and it's slow to decode, so sticking with JPEG was a good idea. Similar issues with WebP, it wasn't good enough to move to.
Hmm I don’t know enough to say, but speed is still important because speed = battery life.
The counterpoint to that is that some things are fast in hardware even if slow in software. H.264 is like this due to some design mistakes but JPEG2K could be the other way round.
